1. Field of the Invention
The present invention relates to an apparatus for filtering liquids, such as fibre pulp suspensions or polluted water.
2. Description of the Prior Art
A common type of filtering apparatuses for filtering liquids comprises at least one annular hollow filter disc adapted to be at least partly immersed in a body of the liquid to be filtered and arranged substantially transverse to an axis extending centrally through the filter disc. The filter disc has two annular opposed walls, which have radially inner ends and radially outer ends. At least one of the two annular opposed walls is pervious by liquid and covered with filter material. Wall means are connected to said opposed walls and form a filtrate chamber, which extends centrally through the hollow disc and which communicates with the interior of the latter. There is means for creating a pressure difference between said body of liquid to be filtered and the interior of the hollow filter disc, such that a fine fraction of the liquid is forced through the filter material into the hollow filter disc and passed therefrom into the filtrate chamber during operation. Fine fraction discharge means is provided for discharging fine fraction from the filtrate chamber and spray nozzle means is provided for spraying a cleansing fluid against the filter material for cleaning the latter. There is means for providing relative movement between the filter material and said spray nozzle means in the circumferential direction of the filter disc.
Conventional filtering apparatuses of this type have their spray nozzle means arranged to spray the cleansing fluid in the form of high pressure jets of liquid from outside the filter disc. The filter disc is vertically positioned and partly immersed in the body of liquid to be filtered, so that part of the filter material is above the body of liquid. The spray nozzle means is stationary and positioned above the body of liquid, while the filter disc is rotatable about said axis. During operation, the spray nozzle means cleans the filter material from particles, as the filter material passes above the body of liquid. The reason for having the spray nozzle means above the body of liquid is to prevent said high pressure jets of cleansing liquid from forcing course particles from the body of liquid into the filter holes of the filter material. Otherwise, the filter material would be rapidly clogged and made inoperative by such course particles during operation.
A drawback to such a conventional filtering apparatus is that some of the small particles which are adhered to the filter material during operation are pushed through the filter holes of the filter material by said high pressure jets of cleansing liquid, with the result that the purity of the fine fraction is degraded. Also, it is inevitable that some particles of relatively course size adhered to the surface of the filter material are pressed into the filter holes of the filter material by said high pressure jets of cleansing liquid, so that, in the long run, the filter material will be more or less clogged. This requires regular stoppage of the conventional filtering apparatus in some filtering applications, for cleaning clogged filter material by chemicals. Another drawback is that only about 60 to 65% of the filter material is operative, i.e. the part of the filter material which is immersed in the body of liquid during operation.